In a wiring structure of a semiconductor memory device, a wiring for transmitting a signal is composed of metal. As integration of semiconductor memory devices increases, a coupling effect between neighboring wiring increases. Therefore, a signal transmitted through the wiring loses an original shape, i.e., it becomes distorted.
As shown in FIG. 1, a wiring structure of a semiconductor memory device according to the conventional art includes a substrate 2 having a device (for example, a transistor, a capacitor) (not illustrated) formed on the upper surface thereof, an insulating film 3 composed of a material such as an oxide film on the upper surface of the substrate 2, and a first conductive film 4 (which serves as wiring for transmitting a signal) formed on the insulating film 3 by a photolithography. A coupling capacitance is generated by a coupling effect with a second neighboring conductive film 4.
As shown in FIG. 2A, it can be seen that there occurs a delay in a signal waveform Q2 at a point P2 in FIG. 1 in comparison with a signal waveform Q1 at a point P1. As shown in FIG. 2B, there occurs a overshooting, that is a distortion of a signal at a point P4 in comparison with a signal waveform Q3 at a point P3. Accordingly, in a semiconductor memory device, an error is caused by the inputting of a signal waveform Q4 at a point P4. For example, inverters (not illustrated) are respectively connected to either end of each conductive film 4, and when an input voltage (Vin) is switched from high level to low level, the input voltage (Vin) is inverted from low level into high level at the point P1. Here, when the voltage (Vcc) is inputted, voltage ground (Vss) must be maintained at the point P3, but there occurs an overshooting due to a coupling capacitance between neighboring conductive films, and at the point P4, the effect is gradually increased, resulting in an error of the inverter receiving an input at P4.
As shown in FIG. 3, the wiring structure of a semiconductor memory device according to another embodiment of the conventional art has a similar structure to that of FIG. 1, which includes a substrate 6 having a device (for example, a transistor, a capacitor) (not illustrated) formed on the upper surface thereof, an insulating film 7 composed of a material such as an oxide film on the upper surface, and conductive films 8,9 formed on the insulating film 7 by photolithography to serve as wiring. The conductive films 8,9 are alternatingly positioned, and ground voltage (Vss) is applied to the conductive film 9 so as to reduce a coupling capacitance between the conductive films 8,9. In the above structure, a distortion of a signal transmitted to the conductive film 8 can be partially reduced. However, the upper surface of the conductive film 8 is not shielded, so a capacitance caused by a coupling between the conductive films cannot be prevented. Therefore, distortion of a transmitted signal remains unimproved.